An N-acetyl-2-aminofluorene (AAF) modified deoxyoligonucleotide duplex, d(Cl-C2-A3-C4-[AAF-G5]-C6-A7-C8-C9)-d(GlO-G1 l-T12-G13-C14-G 15-T16-G 17-G18), was studied by one- and twodimensional NMR spectroscopy. Eight of the nine complementary nucleotides form Watson-Crick base pairs, as shown by NOEs between the guanine imino proton and cytosine amino protons for G-C base pairs or by an NOE between the thymine imino proton and adenine H2 proton for A-T base pairs. The AAF-G5 and C14 bases show no evidence of complementary hydrogen bond formation to each other. The AAF-G5 base adopts a syn conformation, as indicated by NOEs between the G5 imino proton and the A3-H3’ and A3-H2’/H2’’ protons and by NOEs between the fluorene-H1 proton of AAF and the G5-Hl’ or C6-Hl’ proton. The NOEs from the C4-H6 proton to C4 sugar protons are weak, and thus the glycosidic torsion angle in this nucleotide is not well defined by these NMR data. The remaining bases are in the anti conformation, as depicted by the relative magnitude of the H8/H6 to H2’ NOEs when compared to the H8/H6 to H1’ NOEs. The three base pairs on each end of the duplex exhibit NOEs characteristic of right-handed B-form DNA. Distance restraints obtained from NOESY data recorded at 32 OC using a 1 OO-ms mixing time were used in conformational searches by molecular mechanics energy minimization studies. The final, unrestrained, minimum-energy conformation was then used as input for an unrestrained molecular dynamics simulation. Chemical exchange cross peaks are observed, and thus the AAF-9-mer exists in more than a single conformation on the NMR time scale. The NMR data, however, indicate the presence of a predominant conformation (270%). The structure of the predominant conformation of the AAF-9-mer shows stacking of the fluorene moiety on an adjacent base pair, exhibiting features of the base-displacement [Grunberger, D., Nelson, J. H., et al. (1970) Proc. Nutl. Acud. Sci. U.S.A. 66,488-4941 and insertion-denaturation models [Fuchs, R. P. P., & Daune, M. (1971) FEBS Lett. 14,206-2081, while the distal ring of the fluorene moiety protrudes into the minor groove.